Inverters are used to convert an existing direct voltage into an alternating voltage, in order to supply the latter to an alternating current network, for example. In particular in the application in the area of photovoltaics, the highest possible efficiencies are required in this connection.
German Published Patent Application No. 10 2006 010 694 describes a solar inverter, in which two direct voltage sources or solar generators are connected in series, and the connection point forms a first voltage level or a mid-voltage. The two external terminals of the solar generators respectively form a positive and a negative voltage level. Starting from these two levels, the generator voltage is increased above the maximum positive and negative network voltage by a respective step-up converter in the event that the generator voltage is lower than the maximum network voltage. The circuit thus operates with five different, fixed voltage levels. The network phases are then supplied via step-down converters from the respective—in terms of absolute value—next-higher voltage level, the free-wheeling running over the—in terms of absolute value—next-lower voltage level. In addition to the relatively high circuit expenditure, a disadvantage is that in this case the step-up converters and step-down converters produce unnecessary switching losses since in certain operating states the generator voltage is increased to a value from which the actually required voltage level must be generated using a step-down converter. In addition, two solar generators are required to operate this inverter, which must provide as identical a power as possible. This is not always guaranteed, e.g., in the event of a shading of one of the modules.
German Patent No. 10 2007 026 393 describes a solar inverter, in which a step-up converter continually raises a generator voltage to a higher level, but where power is withdrawn from this level in an extended operating mode only when the generator voltage is so low that a storage choke can no longer be magnetized as desired. In this case, the step-up converter operates with a fixed voltage level, from which in turn a step-down converter must generate the currently required voltage level.